This invention relates to vulcanization devices and more particularly to elastomeric vulcanization membranes.
During vulcanization of many elastomeric articles in a mold a device referred to generally as a vulcanizing or curing membrane is used to force the elastomeric article firmly against the mold. In the manufacture of pneumatic tires, for example, a curing membrane seats the uncured tire in the vulcanization mold and retains it until properly cured. Hot fluid such as steam or extremely hot water is circulated within the membrane during the curing process. Heat is transferred from the hot fluid through the membrane to the tire, thereby effecting a vulcanization.
Curing membranes, particularly those used in the vulcanization of pneumatic tires, are often referred to as "bladders" or "water bags". The chief distinction between bladders and water bags or bags is that the former are generally much thinner and are designed to be much more expansible.
Although water bags may contain a fluid under high pressure, they rarely are designed to expand beyond about 5%, whereas a bladder can be stretched up to about 100% circumferentially and up to about 20% laterally or radially. To accomodate expansions, curing membranes are commonly made of some type of elastomeric material. The elastomeric material must be strong enough to withstand repeated pressurizations, expansions and contractions without splitting or otherwise deteriorating.
The cure time of a tire will vary with thickness of the membrane, among other factors. Since elastomeric materials are a relatively poor conductor of heat, a slight difference in the thickness of a membrane can mean a substantial difference in cure time of a tire. In order to reduce the cure time, attempts have been made to reduce membrane thickness. A buckling problem emerges when the membrane thickness is reduced below a minimum point.
Buckling of a tire curing membrane, particularly a bladder is primarily due to the frictional sliding forces between the membrane and the uncured tire as the membrane expands and forces the tire against the mold. Many of these forces are applied to the membrane at a portion associated with the bead area of the uncured tire.
Reducing the thickness of a membrane reduces its strength. Below a given thickness, portions of the membrane cannot carry the stress applied to them, causing erratic expansion and possibly causing portions of the membrane to buckle or crease. This can result in an uneven heat distribution to the tire and thus in a non-uniform cure.
In an attempt to decrease the thickness of a curing membrane while still retaining sufficient strength, specially designed reinforcements can be employed. For example, U.S. Pat. No. 2,695,424 discloses a thin walled curing bag with ribs on its inside surface. In membranes such as disclosed in U.S. Pat. No. 2,695,424, the curing time can allegedly be reduced because of the thinner bag, while the ribs supply the strength that was lost by decreasing the bag thickness. Unfortunately, since the ribs of a curing bag of the type disclosed in U.S. Pat. No. 2,695,424 work essentially independent of each other, a bladder with such a ribbed design can still buckle or kink due to the more extreme expansion of a bladder. A bag of the type mentioned above expands very little compared to a bladder and therefore stress levels are much lower.
To avoid the aforementioned buckling problems, it is desirable to create a membrane with stress carrying capabilities equal in all directions. Some attempts at reinforced curing membranes have employed rectangular rib patterns such as disclosed in U.S. Pat. No. 2,695,424. It can be seen that such a rectangular rib pattern has maximum stress carrying capabilities only in the two directions parallel to the ribs.
Uniform stress carrying characteristics can best be achieved by a ribbed pattern with ribs oriented in as many directions as practical. It may therefore be believed that a pattern of tightly packed circular ribs would be one of the more uniform in stress carrying capability. Circular ribs, however, cannot be arranged in a joined or interconnected manner without leaving non-circular interstices which can create variations in stress carrying characteristics and, even more important, variations in heat transfer through the membrane.